The present invention relates to vehicle navigation systems, and, more particularly, to a system for guiding a vehicle along an optimal route that has been computed beforehand.
Generally, a vehicle navigation system computes from map data the optimal route from a starting point to a destination before starting a journey. En route, the system computes local directions along the optimal route with reference to the vehicle's present location in real time. The system guides the vehicle's navigation from a starting point to a destination by displaying route information in real time on a CRT.
When a vehicle travels, it sometimes deviates from the optimal route. For example, assume a vehicle is traveling on a road in a particular direction. A route guidance display instructs the driver to turn right where the road he is on intersects a second road at right angles. However, the driver cannot make the turn because the second road is under construction. In such a case, the driver is forced to travel straight along the road he is on, without turning to the right at the next intersection. Thus the vehicle has deviated from the optimal route.
Under such circumstances, conventional navigation systems require a manual operation to recompute a new optimal route from a new starting point to the destination. The recomputation takes the vehicle's present location at the time of the manual operation as the starting point.
However, conventional vehicle navigation systems can present a problem when a vehicle is off a new optimal route at the time when or immediately after its computation is completed.
Computation of an optimal route generally requires several seconds. Therefore, when a vehicle keeps on traveling after it has deviated, it moves forward by distance L from the new starting point during the time the new optimal route is computed, where L is given by the equation EQU L=V.times.T,
where V=the velocity of the vehicle and T=the time to compute the new optimal route.
Should the recomputed optimal route require a right-turn at a second intersection where the road currently traveled intersects a third road, the vehicle has passed through the second intersection if the distance between the first and second intersections is shorter than the distance L. As a result, the vehicle is once more off the optimal route, which must be again recomputed in response to a manual operation by the driver. This is a great inconvenience for the driver.
On the other hand, should the new optimal route require driving straight through the next intersection, instead of the right turn required by the original optimal route, the vehicle may turn right at the intersection before the new optimal route is computed, resulting in another deviation.